I'm with Moogs.

Stupid as it is how he did it, Itami got out of the vehicle to attempt to negotiate. It's less intimidating to do so from an apparent equal or weaker position (i.e. not in the vehicle).

Those knights don't understand what the Humvees are, they don't understand how overwhelmingly powerful the SDF is compared, and they don't know about the deal he already made with the princess. He tried to defuse the situation, and when he realized it was getting worse, he had his own people back off to avoid the bloodbath. The blonde bishoujo knight isn't going to kill him, she's gonna take him right back to Pina.

Itami can't risk a single one of his soldiers getting killed. He's on a diplomatic mission, yet a few in his squad are clearly trigger-happy. He loses a soldier to the people they're attempting to make peace with, and his squad won't want to do anything else but kill "the enemy," from there on out. The SDF is in a position of power, they can afford to relent a bit here and there.

Don't forget that the SDF can probably wipe out the entire active military force of this world in a matter of months at most, but that won't get Japan what they want out of this new world. The parallels with recent global conflicts are pretty obvious. De-escalating any conflict they come across is preferable to continually using force.